Keyed automatic gain control



April 15, 1952 A. coTSwoRrH,In;

KEYED AUTOMATIC GAIN CONTROL Filed July 17, 194e" 2 SHEETS--SHEET l ALBERT CoTswoRTH III INVENTOR. 4By #JJM H/.S` AGENT :8 .E .NEL

ohombmm April 15, 1952 A. co'rswoRTH,m 2,593,011

KEYED AUTOMATIC GAIN CONTROL ALBERT CoTswoRTH III N JNVENTOR. M/ u: i BY n a'.

E H/s AGENT in. conjunction Patented Apr. 1-5, 19'52- z,593,o11 i UNITE@ PAT ENT vQFiFIaClE'. y

-lAlbexitntsworthlll, Oak"Pai-k,.`.Ill., :assignor to A,Zenith RaLdio Corporation, .fa .corporation .of

( f'laims. Y A..1 This invention .rela-tes :to television receiving fsystemspandmore particularlygto automatic gain .controlsffor use inisuch systems.

,It .is an. object of ^this `'invention fto provide. an

f automatic kgaincontrol circuit for. use: initelevision receiving systems, zwhich circuit is l.essentially peak operating :and .responsive to :the .tips of i the :synchronizing-,pulses :in television signals .received yby-'such systems. y

Another (object `nf `this invention to ,pro-

Avide tsuch .an :automatic .gain .control .circuit Ywhich has :high 'sensitivity .atthe .inter-vals aof .foccurrence -fofrthe ireceived-v4synhronizing,,pulses `and ,relatively-.flow -`sensitx)ity{during ithelintervals Alli-:tweenthie-individvfailipulses.' 'if' i .AA ,further Yobject :of this vimfenticnis -to .provide:such a circuit `for controlling rthe ygain oa television receiving system, Vwhicvzh.circuitiis.sub- A `stantially unresponsive .to impulse noise signals and the like. y

Yet a-l further .object of `this invention is to .providesuch va circuitin .which `Ithe A.time .constant ,is sufciently vshort to Acompensate for` rapidi` sig' in Lthe -appended gclaims. "The'fjinventionitseli however, together withfurther objects and advantages thereofmayhe'sthe understood'by reiference lto the llowing 'description .when taken with the accompanying drawings in which: v l Y 'Figure i shows a television receiver including oneembodimenttof .the invention, and,

Figure `2 shows such. a;.re ce'iver in which a. second embodimentyf 'the invention4 is incorporated. Referring now .to -Figure il; television v: signals are received on .antenna i, '.and the .received ,sigvnals are V.amplified .in-.radio frequency amplifier stage or stages 2. The amplified signals .from

stages 2 are heterodyned in a usualmixeror iirst vdetector stage r3,randf-.the -;resultinglintermediate v Y frequency ,signals are .ampliedfin i intermediate 4frequency fstagefor .stages 4. I'Ilhecaudio` signals associated-with -zthe A#received televisionwsignals .are

'"2 `tector andiamp'liierstages 5, and 'these detected signals arereproduced signal translating kdevice.

The video signals in vthereceived "television'si'gnal kare .detected `in `a V.second detector vstage vshown in' detail, this stage including a-diode ,detector device f8. The :video `intermediate v.fre- `quencyfsignals `from .the intermediate .frequency amplifier 4 are impressed :between .the cathode of device' :and ground` by means-vof the.usual `coupling -network f9. Detected video signals Vappearing .on .the anode .of diode Y,.8 are Apassed through a usual low-fpasslter netwcrkfij; Y,which network i includes 'sseries connected `incluctances I l -and v12, connected .between 'the 'anode of device "8 fand yoneextremity*fof la terminating re Jsistor l 3, the other extremity :of resistor i3 being :connected Vto ground. The rends` of ,inductances l l and 'l2 `.are coupled "to ground '-by capacitors lI4 and I5rresp'ectively, and the junction of these "inductances is v coultlled to `ground by capacitor 16. `'Ihevalues V(5f/capacitors 14,45 and were `extremely small, `and'the effect thereof may `be obtained @by :the :distributed capacityA of v the detector-circuit.

Theccommon junction `of inductances i Ii fand "I2 is connected to video amplifier stage orstages I1., and Vdetected video'signals .-are-amplied in these stages .and impressed 'in iproperphase von lcontrolel'ectrodelwof `cathode ray receiver;image .tube i9. The-common junction of :inductances ill Vand l2 .is furtherconnected to .signal sepa- .ratorv.stage'20, and :this fstageseparates the .ver- :tical t and `horizontal 'synchronizing signals from =thefdetected video signals "Vertical synchronizing signals from separator 20 are applied to .vertical deflection signal oscillator 4-2-,I .and vertical sweep signals sderived .from .this oscillator Aare impressedzacross' .vertical sweep coil .22 `of ,image 'tube L9. :Horizontal synchronizing signals lfrom fseparator 2D fare used l:to synchronize y horizontal -ideflectionfsignal oscillator 23, `:and thee-sweep V.outiputro'f this oscillator-is applied to the .horizontal deflection coil 240i imageftube J9.

The proposed automatic i gain control is lanla-ptfed-to roperate .on fthe 4tinscf the synchronizing `fpulses of the :received television signals. 4Any change in theflevel of thesetips at .the .second detector voutput :indicates a `change in .signal .strength or in again, .and consequently such changes .may be used `.to Y. correct the gain .of A.the .radioifrequency `andintermediate frequency :an-r- `pliiercircuitsz f-Ihe automatic gaintcc'ntrol .for .the ...above .,de-

- :detectedandfzamplied in @audieireguency de- 5155-scribed receiver.circuitcomprisesiaipairiof .electron discharge devices 25 and 26. Detected video signals from detector L are impressed on control electrode 21 of device 25 through resistor 28. The diode 8 of the detector circuit 1 is so arranged that the detected signals increase in a negative sense as the intermediate frequency carrier amplitudeincreases, and hence the synchronizing pulses contained in the video signals impressed on control electrode 21 of device 25 are negative going, as shown at 29.

The anode 30 of device 25 is directly connected to the positive terminal of a source of unidirectional potential 3|, the negative terminal of which source is grounded. The cathode v32 of device 25 is directly connected to cathode 33 of device 26, these cathodes being connected to ground through common resistor 34. Since the device 25 is cathode coupled to device 26, video signals applied to control electrode 21 of device 25 are impressed on cathode 33 of device 26. The device 25 is so arranged that high noise pulses in the detected video signals are clipped thereby, and signals applied to cathode 33 are as shown in wave form 35, in which it can be seen that the clipped level is slightly above the level of the horizontal synchronizing pulses 36, and a noise pulse 31 is clipped at'this level.

The device 26 functions as a grounded grid amplifier, the control electrode 38 thereof being connected directly to ground, as shown. The anode 39 of device 26 is coupled to the horizontal deflection signal oscillator through a capacitor 40 and a .phase shifting network. The phase shifting network consists of'resistor 4I connected in series with the capacitor 40, and a capacitor 42 connecting the common junction of resistor 4I and capacitor 4|] to ground. Hence, the potential impressed on anode 39 of device 26 is derived from horizontal synchronizing oscillator 23. When the oscillator 23 is of the conventional automatic frequency controlled type, a sine wave 43 may be obtained from the transformer of the synchronized oscillator, the frequency of which sine wave is the frequency of the horizontal synchronizing pulses. However, signals of any wave form, derived from oscillator '23 or signal separator 20 and synchronized to the frequencyof the horizontal synchronizing pulses, may be impressed on anode 39. In the ideal state, these signals would have the form of positive going l pulses having a frequency and width equal to the horizontal synchronizing pulses, so that anode 39 of device 26 is driven positive, and device 26 made effective, solely during the intervals of occurrence of the horizontal synchronizing pulses.

The phase shifter comprising resistor 4l and capacitor 42 is so arranged that when the receiver is synchronized horizontally, the positive crest of the sine wave 43 drives anode 39 of device 26 positive at the same instants when the synchronizing pulses drive cathode 33 of'device 26 negative, and the circuit is such that device 26 is conductive solely during such instants. The

-amount of space current flowing through device f 26 at these instants is dependent upon the level of the synchronizing pulses impressed on cathode 33 and, hence, on the level of the received signals. Hence, a sine wave of potential appears across resistor 44, the average value of which is dependentupon the amount of space current in device 26 and, therefore, on the level of the received signals. This sine wave is filteredV in a filter arrangement consisting of resistor 45 and capacitor 46 connected in series across resistor 3 lil() 4 44, and a control potential is obtained at the junction of resistor 45 and capacitor 46, which control potential is impressed on previous stages of the system, such as stages 2 and 4, by way of lead 41, to control the gain of these stages. It can be seen that the magnitude of this control potential is dependent upon the amount of space current fiowing through vdevice 26 and, hence, upon the strength of the received signals. It can further be seen that this control potential is independent of noise impulses such as impulse 31 rin diagram 35, when such impulses occur at times when the anode potential of device 26 is such that this device cannot be made conductive.

A second embodiment of the invention is shown in Figure 2, wherein like components to the previous embodiment are designated by like numerals. In the circuit of Figure 2 a diode 8 of detector circuit 1 is so arranged that the detected signals increase in a positive sense as the intermediate frequency carrier amplitude increases,

`and hence the synchronizing pulses contained in the detected video signals are positive going. In

the present instance, detected signals from detector circuit 1 are impressed on control electrode 48 of discharge device 49 through resistor 28, these detected signals being of the form shown by wave form 50. The discharge device 44 is connected to form the automatic gain control circuit, the anode 5I of this device being coupled to the horizontal deflection signal oscillator 23 through coupling capacitor 49 and the phase shifting network comprising resistor 4i and capacitor 42, as in the previous embodiment.

The cathode 52 of device 49 is connected to the positive terminal of a source of biasing potential 53, the negative terminal of this source being grounded. Source 53 is by-passed by capacitor 54. Due to the connection to source 53, the cathode 52 is biased positively, and device 49 may be made conductive solely during such intervals when the potential of anode 5l is at a positive peak due to the signal from oscillator 23. Phase shifting network 4|-42 is arranged so that this positive peak occurs in synchronism with the synchronizing pulses on control electrode 48, and hence the tips of these pulses drive device 49 conductive, and as previously a control potential Ais derived on lead 41, the magnitude of this potential being dependent on the signal amplitude during the synchronizing pulses, and this magnitude being relatively independent cf noise signals occurring when the anode 5l is not at a positive potential peak. V

This invention therefore provides an automatic gain control that is essentially immune to noise impulse, and that may be given a short time constant, which short time constant was heretofore impracticable due to excessive Avariation in the automatic gain control potential imparted thereto by the relatively broad vertical synchronizing pulses.

While certain specific embodiments of the inventionhave been shown vand described, it is apparent that modifications may be made without departing therefrom. lIt is intended in the appended claims to cover all such modications as fall within the true spirit and scope of the invention.

I claim:

1. A television receiver for utilizing a composite television signal including video-signalcomponents and recurring synchronizing-signal components comprising: an image-reproducing device and associated scanning elements; a sweep generator coupled to said scanning elements and including means for developing a sinusoidal signal having a frequency corresponding to the repetition frequency of said synchronizing-signal components; a synchronizing circuit for supplying said synchronizing-signal components to said sweep generator to control the operation thereof; a gain control system including a normally non-conductive electron-discharge device having input and output electrodes; means for coupling said output electrodes to said generator to establish a potential difference therebetween which varies in accordance with said sinusoidal signal and tends `to make said discharge device conductive in time coincidence with said synchronizing-signal components; an amplitude limiter for suppressing components of said television signal exceeding the amplitude of said synchronizing-signal components thereof, said limiter being coupled to said input electrodes so that said synchronizing-signal components augment the effect of the potential difference of said output electrodes and render said discharge device conductive during said synchronizing-signal components; a filter circuit coupled to said outi put electrodes for producing a control potential having an amplitude proportional to the amplitude of said synchronizing-signal components; and means for utilizing said control potential to control the gain o-f said receiver.

2. A television receiver for utilizing a composite television signal including video-signal components and recurring synchronizing-signal components comprising: an image-reproducing device and associated scanning elements; a sweep generator coupled to said scanning elements and including means for developing a sinusoidal signal having a frequency corresponding to the repetition frequency of said synchronizing-signal components; a synchronizing circuit for supplying said synchronizing-signal components to said sweep generator to control the operation thereof; a gain control system including a normally nonconductive electron-discharge device having an anode, a cathode connected to a point of reference potential through a resistive impedance, and

a control electrode directly connected to said Number point of reference potential; means for coupling said anode to said generator to vary the potential of said anode withrespect to said reference potential level in accordance with said sinusoidal signal to tend to make said discharge device conductive intime coincidence with said synchronizing-signal components; an amplitude limiter for suppressing components of said television signal exceeding the amplitude of said synchronizingsignal components thereof, said limiter including an electron-discharge device having an anode connected to a source of unidirectional potential, a control electrode for receiving said television signal with said synchronizing-signal components of negative polarity, and a cathode connected to the junction of said first-mentioned cathode and said resistive impedance so that said synchronizing-signal components augment the effect of the anode potential variations of said first-mentioned discharge device and render said device conductive during the peak portion of said synchronizing-signal components; a lter circuit coupled to said anode and cathode of said first-mentioned discharge device for producing a control potential having an amplitude proportional to the amplitude of said synchronizing-signal components; and means for utilizing said control potential to control the gain of said receiver.

ALBERT COTSWORTH, III.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number 2,469,606 Mayle May 10, 1949 FOREIGN PATENTS Country Date France Sept. 4, 1939 France July 15, 1942 Kozanowski June 4, 1946 

